1. Field of the Invention
The invention relates generally to carbon aerogels containing sulfur heteroatom moieties and metal nanoparticles.
2. Description of the Prior Art
Porous carbons are workhorse materials in electrochemistry, featured in electroanalytical, bioanalytical, and electrosynthetic applications, and are used as a conductive binder in fuel cells and batteries. Carbon aerogels, introduced in 1989 (Pekala, J. Mater. Sci., 24, 3221; all referenced publications and patents are incorporated herein by reference), are another class of porous carbon with demonstrated advantages for applications in supercapacitors and capacitive deionization. The morphology of these high-surface-area materials may be tailored by varying synthetic conditions such as the R/C ratio of resorcinol (R) to the polymerization catalyst (C), the reactant weight fraction, the curing strategy, and the pyrolysis conditions. Additionally, carbon aerogels represent a promising alternative to conventional carbon blacks for fuel-cell-catalyst supports because of their through-connected mesoporous/macroporous network for fuel and oxidant transport, monolithic architecture with 3-D electronic connectivity, and large available surface area for the dispersion of catalysts such as those based on Pt group metals.
Carbon aerogels have previously been investigated for fuel-cell applications; in particular, carbon-aerogel-based gas-diffusion electrodes were prepared for use in proton-exchange membrane (PEM) fuel cells (Petricevic et al., Carbon, 39, 857 (2001); Glora et al., J. Non-Cryst. Solids, 285, 283 (2001). In these studies, membrane-electrode assemblies (MEAs) were prepared by sputtering a Pt catalyst layer onto thin films of carbon-fiber-reinforced carbon aerogels. Although a functional H2/air fuel-cell stack was produced, the power density was a factor of six lower than commercially available electrodes.
Several research groups have devised strategies to prepare metal-modified carbon aerogel composite materials. Dunn and coworkers used chemical vapor deposition to incorporate ruthenium nanoparticles within a carbon aerogel (Miller et al., Langmuir, 15, 799 (1998); Miller et al., J. Electrochem. Soc., 144, L309 (1997)). Electrochemical oxidization of the ruthenium to hydrous RuO2 (RuOxHy) significantly improved the specific capacitance relative to the carbon aerogel alone, demonstrating the electrical addressability of the Ru/RuOxHy deposits and the usefulness of metal-modified carbon aerogels for electrochemical applications. Erkey and coworkers developed a supercritical deposition method to incorporate Pt complexes into carbon aerogels, where said complexes could be converted to Pt metal nanoparticles by subsequent thermal processing (Saquing et al., J. Phys. Chem. B, 108, 7716 (2004). Additional methods for incorporating metals into carbon aerogels include metal-ion electrosorption and inclusion of noble metal salts in the initial resorcinol-formaldehyde reaction mixture.